The growth of a pine tree is examined by preparing “tree cookies” (cross-sectional disks) between whorls of branches. The use of Christmas trees allows the tree cookies to be obtained with inexpensive, commonly available tools. Students use the tree cookies to investigate the annual growth of the tree and how it corresponds to the number of whorls on the tree. Height–diameter and height–age relationships are also investigated. While aimed at high school classes, the procedure can be adapted for younger students.
The study of forestry can be linked to the biology curriculum at many levels. Project Learning Tree (http://www.plt.org) has been connecting forestry to the elementary and secondary curriculum for 30 years. The Wood Magic curriculum (http://woodmagic.vt.edu; Zink-Sharp & Davis, 2008) presents several activities for elementary through high school. Previous articles in The American Biology Teacher have used forestry to study ecology in high school (Varbalow, 1963; Glenn, 1995; Rubino & McCarthy, 2002) and interdisciplinary studies in middle school (Zipko, 1983).
Most students have at least a cursory knowledge of trees – even urban environments have parks and street trees. Although some students may have made a leaf collection, most have never had the opportunity to study trees in depth.
Pine trees are ideal for studying tree growth. Not only is the wood easy to work, but discarded Christmas trees are plentiful (and free!) after the Christmas season. Such trees are relatively young and in a rapid-growth phase, so the annual rings are wide and easy to measure. Additionally, because Christmas trees are grown for their symmetry, their cross-sectional disks are usually symmetric, unlike wild-grown trees that may have been subjected to asymmetric environmental factors such as slope, crowding, or uneven light availability.
The objective of this laboratory investigation is for students to become familiar with how a tree grows, and it is targeted for high school students. Elementary school students may simply learn that a tree makes a new ring of wood each year, and that the age of a tree can be determined by counting rings. Older students can investigate growth rates and wood structure. Advanced classes can get into the details of wood structure or explore dendrochronology (see “Extensions” at end of article).
The aboveground portion of a tree grows in two ways. The tree canopy increases in height and diameter as the terminal buds at the tips of each twig grow into new twigs and leaves in the spring and summer (Zink-Sharp & Davis, 2008). The trunk increases in diameter as the cambium between the bark and the wood produces new phloem (inner bark) and xylem (new sapwood) (Zink-Sharp & Davis, 2008).
A better method to get the age of a tree is to count the rings in a cross-section of the trunk. In the spring, the cambium produces “earlywood,” with large cells and thin cell walls. As the season progresses, the cambium produces “latewood” with smaller cells and thicker cell walls. Earlywood looks light, whereas latewood looks dark (Zink-Sharp & Davis, 2008). The abrupt transition to earlywood in the spring produces the sharp contrast that makes the growth ring visible (Figure 3).
Dendrochronology, the study of the growth of tree rings (Laboratory of Tree-Ring Research; Zipko, 1983; Rubino & McCarthy, 2002), is used by a wide range of occupations. Foresters use data from tree rings to determine the age and condition of a timber stand. University researchers use the data to study many aspects of forest ecology, including the archaeological investigation of wooden structures ( Judson, 2007) and to document past climate change (University Corporation for Atmospheric Research, 2005).
Professional foresters and forest researchers use an increment borer to obtain cross-sectional samples (“cores”) without damaging a living tree. However, these are expensive instruments and are not suitable for use by younger students. The procedure described below uses only inexpensive and commonly available tools.
Materials & Equipment
One recently cut pine tree (older trees can be used, but knowing the approximate date of harvest is useful for multi-year studies)
Fine-tipped permanent marker (e.g., “Sharpie”)
Sandpaper in a range of grits from coarse (60) through medium (100–250) to very fine (300–400)
Power sander (not essential, but very useful)
Measuring tape, 4 to 5 m long
Flexible measuring tape (e.g., the type used in sewing), 1 to 2 m long or a “dbh” (diameter at breast height) tape that reads diameter, rather than circumference
Care should be taken when using saws and pruners. Sanding may cause abrasions if not handled properly. Proper personal protective gear must be worn when using power equipment: safety glasses, hearing protection, dust mask, long hair tied back, long sleeves secured at wrists.
Measurements will be easier if the branches are removed first. Care should be taken near the top of the tree to remove the branches and not the top. Students should do all the measurements. Sawing of the tree cookies can be done by older students. In younger-age classrooms, the teacher should do the sawing.
Measure and record the distance of each whorl from the base of the tree.
Measure and record the circumference of the trunk between each whorl and the next and at the base. Divide.
Before cutting up the tree further, observe changes in bark between older and younger parts of the tree (Figure 4). Note that the transition in bark is abrupt and occurs at a whorl.
Cut a “tree cookie” (cross-sectional disk) approximately half-way between each whorl and the next. It should be 2 to 3 cm thick for easier handling. Label each cookie with a permanent marker, starting with A at the base of the tree and proceeding through the alphabet.
Let the tree cookies dry for a week or two before proceeding. Recently cut trees contain up to 50% water by mass. Dried tree cookies are easier to sand, and the rings will be easier to read. It is common for tree cookies (especially larger ones) to split as they dry.
Sand the surface of each tree cookie to enhance the rings. Start with a coarse grit, such as 60.
Finish with a very fine grit, such as 400. It is not necessary to sand both sides of the tree cookie (Figure 5). If time is a factor, finishing with 220 grit gives acceptable results.
Count the rings in each tree cookie: Count each transition from dark latewood to light earlywood. The outermost ring is wood produced in the year the tree was harvested.
Scan the cookies into a digital image file.
Measure the width of the rings on each cookie.
Examine the cookies under a dissecting microscope.
It is easy to “divide and conquer” data collection by distributing the rough-cut cookies among students and having one or two students work on each cookie. The completed cookies can be scanned into digital photo files, enabling each student to work with a print-out to prepare his or her own analysis of the entire set. Although the analysis can be done with just the scans, loss of sensory factors – texture, odor, heft – makes for a less satisfactory experience.
Height and diameter measurements and cutting the tree cookies takes 60–90 minutes. Sanding 20 tree cookies with a power palm sander takes 60–90 minutes. Preparing the spreadsheet and data analysis takes 30–60 minutes. Microscopic examination of the samples takes 15–60 minutes.
Suggested Class Schedule (50-minute classes)
Day 1: Measure and prepare tree cookies.
1 to 2 weeks later:
Day 2: Sand tree cookies (distributed among students) and prepare spreadsheet.
Day 3: Analyze data: count rings, prepare charts, discuss.
Day 4: (Optional) examine tree cookies with dissecting microscope.
Teaching about tree growth and structure can occur while waiting for the tree cookies to dry. “Trees: Recorders of Climate Change” (University Corporation for Atmospheric Research, 2005) has good suggestions for having students learn about tree rings.
Once the tree cookies have been prepared, data analysis can be initiated by the teacher or, for an inquiry-based lesson, class discussion can develop hypotheses and determine the data and analyses that are needed. Data can be entered into a spreadsheet to facilitate analysis (Table 1) – although graphing and analyzing by hand is a useful exercise. Statistical analysis can be used as appropriate.
Suggested plots include these:
Height against whorl number (Figure 6A). My data from a 2011 Virginia pine Christmas tree show quite uniform growth, with a spacing of 15 to 16 cm per whorl on average.
Diameter against whorl number (Figure 6B). My data show less uniformity, but again a relatively linear decrease of ∼0.3 cm per whorl.
Number of rings against the whorl number (Figure 6C). My data show that the assumption of one whorl of branches for each year of growth is not supported – at least for young Virginia pines.
Diameter against height (Figure 6D). My data show some variability, but a relatively linear decrease of diameter with height.
Examination of the tree cookies under a dissecting microscope is well worth the extra time. First, you can notice how much more information you can get from the sanded side compared to the rough-cut side. If tree cookies from other species are available, differences in wood structure are apparent (Hoadley, 1990; Zink-Sharp & Davis, 2008). Sample tree cookies are available from Forestry Suppliers (http://www.forestry-suppliers.com). A local sawmill may be willing to provide scraps from several species. Advanced classes can relate the differences in structure to differences in wood properties and tree morphology and physiology (Hoadley, 1990; Zink-Sharp & Davis, 2008; Wojtech, 2011).
Although this activity is aimed at high school students, it is easy to adapt to other age groups. Obviously, elementary school students would not participate in sawing and sanding, but measurement and simple graphing are part of the elementary school math/science curriculum.
There are many ways to extend this project. The class could compare several different trees, both additional trees of the same species and different conifer species. Although most Christmas trees are pines, they may also be spruce, fir, juniper, or cedar, depending on local availability.
Students could also compare conifer growth with that of a hardwood tree. Because hardwoods do not produce whorls of branches, the trunk should be sampled at a regular interval (such as every 25 cm for a small tree, or 1 m for a large tree). Hardwoods are more difficult to work with – power tools are almost essential, and the time required to get a satisfactory finish to the wood is longer. Hardwoods have a wood structure that is very different from that of conifers (Hoadley, 1990; Zink-Sharp & Davis, 2008). Some of this structural difference is apparent to the naked eye, but examination under a dissecting microscope is preferred. I prefer to have students view and sketch what they see before learning about the specific features revealed under the dissecting microscope (pore arrangements, rays). Students who enjoy this microscopy could use a hand lens to view solid wood samples in their homes: furniture, kitchen cabinets, cutting boards, and doors can provide fascinating opportunities to attempt wood identification. Even lump charcoal still shows wood structure. Hoadley (1990) is an invaluable resource for this work.
Tree cookies from larger trees are much more difficult to work with. They are usually cut with chain saws that leave marks that require much more work to sand off, requiring at a minimum a belt sander, and possibly a power planer. An increment borer is a better choice for sampling large trees. (See Rocky Mountain Tree Ring Research reference for detailed information on use and care of increment borers.)
Samples can be varnished and preserved for a multi-year comparison. This is especially useful if subsequent years’ trees come from the same location. Dendrochronology techniques (see the following references below: The Ultimate Tree-ring Web Pages; Rocky Mountain Tree Ring Research; Laboratory of Tree-Ring Research) can be used to investigate the relationship between annual weather (temperature, rainfall) and ring size and structure. Sample tree cores might be available from a local forester, university forestry department, or your local cooperative extension agent.
Bark is a subject all its own. Wojtech (2011) presents not only an in-depth description of bark form and growth, but also a field guide to bark at different ages for many species found in the northeastern United States.
And finally, the more artistically inclined students may find inspiration in prints made from tree cross-sections (Gill, 2012).
Glossary for Growth of a Pine Tree
Cambium: the layer of dividing cells that surrounds the trunk between the phloem and xylem. New cells are formed here as the tree grows.
Candle: the new growth at the tips of branches that typically points upward and resembles a candle sitting on the branch. See Figure 1 for an illustration.
Conifer: a tree that produces cones, such as pines, spruces, and firs; softwoods.
Hardwoods: trees that have broad leaves, such as oaks and maples. Not all hardwoods have wood that is “hard.” For example, yellow-poplar (Liriodendron tulipifera) has wood that is almost as easy to work as pine.
Picture post: a post topped by a small shelf with a raised octagonal center that points to the eight major compass points (N, NE, E, SE, S, SW, W, NW). This facilitates positioning a camera pointing at each of these directions to document landscape changes over time. For more information, see picturepost.unh.edu.
Softwoods: trees that have needle-like leaves, such as pines, spruces, and firs; conifers.
Whorl: a set of branches attached around the trunk at the same height on the tree.
Materials are in the Extras-PineTree-Web folder, available online as a ZIP file: http://dl.dropbox.com/u/62299864/EXTRAS-PineTree-ms.zip .
Video of 1 year’s growth of a pitch pine.
Video of 3 years’ growth of a pitch pine.
Digital scan of all 20 tree cookies.
Excel file with sample data and charts.
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